The invention concerns an optically variable element which at least in surface portions has an interface which is preferably embedded between two layers of a layer composite and which forms an optically effective structure which spatially projects and/or is set back with respect to a (notional) reference surface, wherein the optically effective structure has at least one free-form surface appearing three-dimensionally for a viewer in the form of an alphanumeric character, a geometrical figure or another object.
Optically variable elements of the above-described kind are used for example as security elements for authenticating or identifying value-bearing documents, for example banknotes, cheques, etc, identity cards and passes, credit cards or other articles to be safeguarded. Such optically variable elements are also already used for decorative purposes, in which respect the boundary between use as a security element and use as a decorative element is frequently fluid. In that respect a particularly frequent requirement is that security elements also have a certain decorative effect, which applies for example when the situation involves guaranteeing the authenticity of certain articles, for example cigarettes, valuable cosmetic preparations and so forth, by corresponding elements.
For use as a security or decorative element, the known optically variable elements are generally applied to the corresponding substrate in the form of transfer films, in particular hot stamping films, or in the form of laminating films, in which case the interface forming the optically effective structure is then provided between two corresponding lacquer layers. In the case of transfer films those lacquer layers are part of the decorative layer arrangement which can be transferred from the carrier film on to the substrate, wherein instead of a lacquer layer it is also possible to provide an adhesive layer or the lacquer layer may have adhesive properties. In the case of laminating films the interfaces are in principle produced in the same way. The difference between laminating and transfer films however is that, in the case of laminating films, the lacquer and possibly adhesive layers serving as the decorative element remain on the carrier film when the laminating film is applied to a substrate. Finally it is also conceivable for packaging or decorative films to be basically like laminating films, but for those films, for example for packaging purposes, to be used as such without being laminated on to a substrate.
In this connection it is also already known for three-dimensional effects to be produced by way of suitable structuring of the interface between two layers, in particular lacquer layers, or in relation to air. For example cheque and credit cards are known, in which certain objects appear in different positions or perspectives in dependence on the viewing angle, or the impression is given to the viewer as though the corresponding object were standing three-dimensionally out of the surface of the carrier for the optically variable element.
Hitherto those three-dimensional effects were generally produced holographically, in which respect that procedure has on the one hand the disadvantage that a comparatively high level of apparatus expenditure is involved in production of the masters required for replication in corresponding layers. In addition holographically produced structures also suffer from serious optical disadvantages. In particular their shine is frequently defective. In addition, there is generally no possible way of increasing the attractiveness of a correspondingly optically variable element by achieving certain colour effects.